1. Field of the Invention
This invention relates to a contact device for conveying electric current to an article.
2. Description of the Prior Art
A commonplace requirement for metallurgical and other purposes is to heat a billet of metal. This is frequently done by passing a heavy current through the billet from one end to the other exploiting the effect of direct resistive heating. A difficulty encountered is to pass the current into and out of the billet. Conventionally this has been achieved by complicated water-cooled copper clamps which have the disadvantages of being expensive and of tending to mark or even damage the billet whilst still failing to ensure uniform contact over all the end surface of the billet.
Another commonplace requirement is to heat wire. Direct resistive heating is sometimes employed for this purpose with the current being passed into and out of the wire through copper drums over which the wire slides but this can lead to marking or damage of the wire surface due to arcing and therefore more often the furnaces are used, which involves much capital cost and some waste of heat.
It has been proposed in U.S. Pat. No. 3,060,304 to provide a method of heating an article by immersing the article in a heated bed of fluidised carbon particles, the article being heated mainly due to Joule's heat arising from the high contact rsistance between the fluidised electroconductive powder and the surface of the article. The apparatus used to carry out this method incidentally acts to provide an electric contact with the article to be heated and comprises a current feeder member adapted to contact a fluidised bed of carbon particles, means for fluidising the particles and means for maintaining a part of said article, spaced from the current feeder member, in contact with the fluidised bed of articles.
The applicants have found that when using carbon particles to introduce electric current into a static article needles of coagulated carbon particles tend to form in the fluidised bed leading to deterioration in the efficiency of the device. It is thought that the needles are due to localised overheating of the bed arising from firstly a "pinch effect", since when electric current flows through the particles there is a tendency for the particles to be squeezed together and thus form a concentrated current path, and secondly because carbon has a negative temperature co-efficient of resistance so that as the temperature of the carbon particles increases their resistance decreases thereby inreasing the current flow and so increasing the temperature with a consequent further increase in current flow and so on. These two effects are thought to give rise to the formation of the carbon needles.